Iran has stepped up its production of high-grade enriched uranium and has re-landscaped one of its military bases in an apparent effort to hamper a United Nations inquiry into the country's nuclear program, a U.N. report said Thursday.

Iran's facilities have produced 189.4 kilograms (417.6 pounds) of the enriched uranium, an increase of 43.8 kilograms (96.6 pounds) since the last U.N. analysis in May, the report said.

That enriched uranium is produced at declared facilities in Iran that don't include the military base cited in Thursday's report.

Iran hasn't allowed the agency access to the military site, called Parchin, and, in fact, has been conducting "extensive activities" at that location "that will significantly hamper the Agency's ability to conduct effective verification," the U.N. International Atomic Energy Agency said in its report.

"Significant ground scraping and landscaping have been undertaken over an extensive area at and around the location, with new dirt roads established," the report said.

Iran must respond to the agency's concerns "without further delay." Otherwise, the U.N. body won't be able to resolve issues about Iran's nuclear program and "the existence of possible military dimensions" to the program, the report said.

Many Western diplomats and nuclear experts believe the Parchin site has been secretly used to test high-explosive nuclear triggers, an essential step toward achieving a weapons capability. Iran denies that Parchin has any role in its nuclear program, which it insists is for peaceful purposes.

A meeting in Vienna last week between IAEA officials and an Iranian delegation, which in part was to discuss access to the Parchin site, ended without progress.

Iran acknowledges that it is increasing its number of centrifuges to enrich uranium. President Mahmoud Ahmadinejad said last month that 11,000 centrifuges were now operational, about 1,000 more than estimated by the IAEA in May.

In a letter Wednesday to the agency, Iran said the allegations of nuclear activities at the Parchin site were "baseless," the report said.

But the IAEA reasserted its ongoing alarm about Iran in Thursday's report.

"Since 2002, the Agency has become increasingly concerned about the possible existence in Iran of undisclosed nuclear related activities involving military related organizations, including activities related to the development of a nuclear payload for a missile," the report said.